Process for making a metallurgically slow reacting mold

ABSTRACT

The mold is made by initially dipping a pattern into a mixture of pulverized magnesium carbonate and colloidal hydrolyzed silicic acid to form a number of layers thereon. The green mold is then burned for a length of time and at a temperature to cause the formation of forsterite (Mg2SiO4) whereby the active magnesium oxide (MgO) formed during burning completely bonds in the silicic acid. The silicic acid cannot then detrimentally effect the precision casting of steels or alloys having a high chromium, titanium and/or aluminum content, especially at high casting temperatures.

United States Patent 11 1 Schneider [11] 3,815,658 [451 June 11, 1974 15 PROCESS FOR MAKING A METALLURGICALLY SLOW REACTING MOLD a lnventori Hans Schneider, Winterthur,

Switzerland Assignee: Sulzer Brothers Ltd., Winterthur,

Switzerland Filed: Apr. 3, 1972 Appl. No.: 240,531

Related US. Application Data Continuation-impart of Ser. No. 9,265, Feb. 6, 1970, abandoned.

[30] Foreign Application Priority Data Mar. 13, 1969 Switzerland 3776/69 US. Cl 164/35, 164/26, 164/41 Int. Cl. B220 9/04 Field of Search 164/23, 24, 25, 26, 41,

References Cited UNITED STATES PATENTS 3,148,422 9/1964 Payne 164/26 3,206,810 9/1965 Hockin et a1. 164/26 3,587,713

Primary Examiner.l. Spencer Overholser Assistant Examiner-John E. Roethel Attorney, Agent, or Firm-Kenyon & Kenyon Reilly Carr & Chapin [5 7] ABSTRACT The mold is made by initially dipping a pattern into a mixture of pulverized magnesium carbonate and colloidal hydrolyzed silicic acid to form a number of lay- 7 Claims, 1 Drawing Figure 6/1971 Halsey ..164/26X v PATENTEDJun 1 1 1914 INVENTOR.

PROCESS FOR MAKING A METALLURGICALLY SLOW REACTING MOLD This application is a continuation-in-part of U5. application Ser. No. 9,265 filed Feb. 6, 1970 now abandoned.

This invention relates to a mold and process for constructing a casting mold. More particularly, this invention relates to a process for making a metallurgically slow reacting casting mold, especially for precision casting.

In precision casting, use is made of a lost pattern technique in which a pattern for a casting to be produced is initially covered with one or more layers of a mold mass consisting of a binding medium and a finegrained refractory material so as to form a casting mold. This mold is then subjected to a step in which the base pattern is removed by being melted,-burned or dissolved out-of the mold. Since use is predominantly made of burned molds, the mass for the production of the mold is preferably made of a binding medium containing silicic acid, because silicic sols have proved to be the best binding medium. For example, it is known through the hydrolysis of a silicic ester, e.g. ethyl silicate, to produce a colloidal silicic acid solution, and to mix this as a binding medium with fine-grained refractory mold material.

However, in using binding mediums containing silicic acid, it has been show that any silicic acid contained in the finished mold can enter into undesirable reactions during the casting of ceratin metals and alloys, particularly at higher casting temperatures. Consequently, in order to avoid this defect in casting, it has been necessary to bond in the silicic acid contained in the mold mass or in the binding medium in a stable manner. For example, in the production of a mold made from a refractory material, such as corundum, mullite or magnesia, a binding medium that consists of a mixture of magnesium oxide, aluminum oxide and/or zirconium oxide and of a colloidal silicic acid solution has been added to the mold material in order to stabilize the silicic acid.

Further, it has now been found in practice that for steels or alloys having a very high content of chromium, titanium and/or aluminum, for example for alloys having a chromium basis and which are cast at relatively high temperatures of, for example, above 1,700 C, molds made by the usual known process do not have the chemical stability necessary to bond the silicic acid in stable fashion.

While it is recognized that the only silicic acid compound which is stable at high casting temperatures with respect to the above metals is forsterite (Mg SiO such, however, is formed at relatively low temperatures, e.g., at about l,000 C only from chemically active magnesiumoxide (MgO) and silica gel. Further, because active magnesium oxide has a gelling action on colloidal silicic acid, the preparation of a dipping mass directly from active magnesium oxide and colloidal silicic acid whether in an aqueous or alcoholic suspension is not possible. Therefore, a dipping mass made directly from these two'constituents does not have sufficient stable time to allow the application of the required layers on a suitable mold pattern without gellin liccordingly, it is an object of the invention to stabilize silicic acid in a dipping mass for a mold.

lt is another object of the invention to utilize forsterite in making a mold for precision casting.

Briefly, the invention provides a mold and a process for making the mold in which, at least the layer delimiting a hollow casting space, has a significant proportion of forsterite. This layer or layers can also include inactive refractory filler substances.

ln forming the mold, use is made of pulverized magnesium carbonate as amold material with colloidal hydrolyzed silicic acid mixed in as a binding medium. After forming a green casting mold with this material as a coating thereon, the casting mold is burned so that magnesium oxide and forsterite are formed from the magnesium carbonate and silicic acid. Thus, the proportion of magnesium carbonate in the mold mass is made at least large enough for the active magnesium oxide produced therefrom to be stochiometrically sufficient to bond the added silicic acid completely in the forsterite. By completely is meant that the silicic acid should be bound to an extent to prevent macroscopically discernible reactions with and within the mold during casting of chromium based alloys in preheated molds at casting temperatures'of approximately 1,700 C and above. A minimum of 99 percent of the silicic acid should be converted into forsterite. The ratio of the quantity of magnesium carbonate to the quantity of silicic acid in the mold mass, relative to the silicon oxide content of the silicic acid, is therefore about 10 to one and within a range of from three to one to 12 to one; while the proportion of forsterite in the layer of the burned mold is approximately 30 to 40 percent. The remaining constituents present in the layer include magnesium oxide and, on occasion, other inactive refractory filler substances.

The chemically inactive refractory filler substances which may be used are those tillers which do not react with the silicic acid during the burning of the mold and which do not react with the molten metal being cast during the casting operation. Such filler substances include inactive chemically stable magnesium oxide, periclase and zirconium oxide.

The sanding medium to be used for the sanding of the individual layers can advantageously be made of silicon carbide (SiC) since this is chemically inert andhas great heat conductivity and small heat-expansion. I

In order to obtain smooth surfaces with casting of the i above type, it is merely necessary that the mold layerdelimiting the hollowv space for the casting be made of the moldmass according to the invention. The further layers may then, as is known, be produced from zirconium silicate, mullite or other refractory mold materials. It is of course also possible to produce all the layers of a mold by the process of the invention.

These and other objects and advantages of the invention will become more apparent fromthe following detailed description and appended claims taken in conjunction with the accompanying drawings in which:

The sole FlGURE illustrates a cross-sectional view of a mold according to the invention.

The invention will be explained in more detail in the following by the aid of two examples.

EXAMPLE I A dipping mass was produced by mixing 5 kilograms (kg) of fine-grained pulverized magnesium carbonate and 2.4 liters of hydrolyzed ethyl silicate (SiO. content some 12 ml 6 percent). The mixture is stable for about 6 hours time.

In order to produce a mold, a pattern corresponding to the casting desired was made from a destructable material e.g.,"urea. The pattern was then dipped into the mold dipping mass producing which mass left a thin coating clinging to the pattern surface. The surface was then covered over-with a flowable coarse refractory material, for example, silicon carbide or mullite, and allowed to dry for about 2 hours. This dipping and covering, with subsequent drying, was repeated some seven to 10 times. 1

The shell-like mold element obtained was then, for the purpose of detaching the urea pattern, placed in a waterbath, following which the mold element was dried in the usual way. The pattern can be dissolved out completely by the aid of the solvent entering through the pores of the mold element; or else it can be dissolved out only partly,so that the rest of the pattern left behind becomes destroyed during the following burning of the mold. This burning is done at a temperature of from 800 to l,000 C with a maximum temperature of about 1,000,C, and for a period of time of from 8 to 12 hours. The firingtemperature and the duration of firing are not independent of one anothenThat is, they are inversely proportional to each other. This means that with higher firing temperatures, shorter firing times are required and vice versa. The finished mold was characterized by an excellent surface quality of the hollow casting space and by very good strength characteristics.

During burning, the magnesium carbonate disassociates into active magnesium oxide and carbon dioxide. The carbon dioxide escapes, while the active magnesium oxide with the silicic acid present forms forsterite so that the silicic acid is bonded in, in stable fashion. During the further burning, the excess of magnesium oxide which is formed is converted into crystal form, i.e., periclase, which is resistant to water and other aqueous solutions.

Because of the thermal dissociation of the magnesium carbonate during burning, the mold loses a part 'of its mechanical strength and resistance 'to abrasion and ages: 70 percent chromium, 29 percent cobalt, up to 0.5 percent yttrium, the remainder iron and aluminum along with the usual impurities. The casting temperature was approximately l ,750 C. The casting obtained was characterized by an immaculate surface.

As has. already been mentioned, under certain circumstances, the pattern can be coated with only one layer of the mold mass made as described above, and

Y can then be dipped into one of the mold masses of the aforesaid known composition. It is, however, also possible, after applying a layer consisting of magnesium oxide and forsterite by the aid of the process of the invention to then back up this layer. in the usual way in a mold-box, with a suitable second mold mass, for example, with a cement-bonded mold mass.

EXAMPLE ll 1 kilogram (kg) of pulverized chemically pure magnesium carbonate percent pure) was mixed with 1.25 liters of binding medium, produced from hydrolyzed ethyl silicate having a 14 percent content of SiO To this mixture was added, as an inactive filler substance, l kilogram (kg) of pulverized chemically pure periclase (which is, as is well known, a chemically stable crystalline form of magnesium oxide) also .95 percent pure. The mold mass obtained in this way was likewise stable for 6 hours. t

The further layers for the production of the mold of the invention correspond to those of Example]. A

A burned mold according to Example ll was found to have a composition of about 50 percent periclase and 40 percent forsterite, while the remainder consisted of another stable-form of magnesium oxide.

Referring to the drawing, the burned shell mold consists of two layers and is to be used for casting a turbine blade. The mold cavity 1 is composed of three parts. Part la has the shape of the turbine blade itself; part 1b forms the blade root after the casting; and part 1c serves as a riser for pouring. According to the invention, the inner layer 2 delimiting the mold cavity 1 consists of chemically inactive magnesium oxide up to 60 percent of forsterite (Mg 'SiO up to 40 percent. Further, this layer 2 may contain silicon carbide as a sanding medium. The outer layer 3 imparting the mechanical strength to the mold is made of silicates of zirconium and/or aluminium, for example molochite, and of silicon carbide as a sanding medium.

The pulverized materials used for the mold mixture have grain sizes form 0.005. to 0.05 mm.

' Steps can be taken to reduce the firing temperature and/or the firing time within the mentioned ranges of from 800 to l,00O C and from 8 to 12 hours. First, the grain sizes of pulverized materials can be reduced.

Further for the reaction occurring between magnesium oxide and silica on firing, a catalyst can be added to the molding material. Such a catalyst enables the firing temperature to be reduced and promotes the reaction to form forsterite also.

What is claimed is:

l. A process for producing a metallurgically slow reacting mold including the steps of mixing together pulverized magnesium carbonate as a mold material and colloidal hydrolyzed silicic acid as a binding medium to form a dipping mass, dipping a destr'uctable pattern into said dipping mass to coat said pattern with a layer of said mixture and form a green casting mold; and subsequently burning the green casting mold for a length of time of from 8 to 12 hours to remove said pattern and to form a magnesium oxide and forsterite mold from said layer wherein the quantity ratio of magnesium carbonate to silicic acid in said dipping mass and the length of time of said burning are sufficient to completely bond the active magnesium oxide formed during said burning with said silicic acid as said forsterite.

4. A process asset forth in claim 1 which further comprises the step of impregnating said mold element with colloidal silicic acid.

5. A process for producingametallurgically slow reacting mold comprising the steps of mixing together pulverized magnesium carbonate as a mold material and colloidal hydrolyzed silicic acid as a bonding medium to form a dipping mass wherein the quantity ratio of magnesium carbonate to silicic acid amounts to about to one; producing a coating of said mass on a destructable pattern to coat said pattern with at least one layer of said mass; forming a green casting mold with said layer forming an inner surface thereof; and subsequently burning the green casting mold a length of time from 8 to 12 hours to completely bond the active magnesium oxide formed during burning with said silicic acid to form a magnesium oxide and forsterite mold from said layer. 6. A process for producing a metallurgically slow reacting mold comprising the steps of mixing together pulverized magnesium carbonate as a mold material and colloidal hydrolyzed silicic acid as a bonding medium to form a mixture wherein the quantity ratio of magnesium carbonate to silicic acid is sufficient to have the active magnesium oxide produced upon subsequent subjection of said mixture to burning completely bond said silicic acid as forsterite,

producing a coating of said mixture on a destructable pattern with at least one layer of said mixture thereon,

forming a green casting mold of said coating and pattern, and

thereafter burning the green casting mold to remove said pattern and form a magnesium oxide and forsterite mold from 8 to 12 hours from said layer, said burning being of a length of time sufficient to completely bond said silicic acid in said active magnesium oxide produced during said burning step.

7. A process for producing a metallurgically slow reacting mold consisting of mixing together pulverized magnesium carbonate as a mold material and colloidal hydrolyzed silicic acid as a binding medium to form a dipping mass; and

dipping a destructable pattern into said dipping mass to coat said pattern with a layer of said mixture and form a green casting mold;

subsequently burning the green casting mold at a temperature of from 800 to l,000'C for a length of time of from 8 to 12 hours to remove said pattern and to form a magnesium oxide and forsterite mold from said layer, wherein the temperature and burning time are inversely proportional to each other and wherein the quantity ratio of magnesium carbonate to silicic acid in said dipping mass and the burning are sufficient to completely bond the active magnesium oxide formed during burning with said silicic acid as said forsterite.

UNITEK) STAiEfi YATENT ()FFMIE I CERT! FHIAT?) {5E CURRWQ'E'EQN Patent 2,81m6s8 Dated June 11. 197M Inventor(s) V Hans Schneider It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 29, "show" should be --shown-=-.

Column 1, line 31, "ceratin" should be --certain--.,

Column 4*, line 41, "form" should be -fro1rno Column 6, line 8, after "mold" insert --from 8 to 12 hours.

Column 6, lioe-lO, after "mold" delete "from 8 to 12 hours--.

Signed arld sealed 'this 1st day of October 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C, MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-1Q5O (10-69) USCOMM-DC 603764 69 i U.S GOVERNMENT PRINTING OFFICE: 1969 0-356-334. 

2. A process as set forth in claim 1 wherein said ratio of magnesium carbonate to silicic acid is from three to one to 12 to one.
 3. A process as set forth in claim 2 wherein said burning occurs at a temperature of from 800* to 1,000* C.
 4. A process as set forth in claim 1 which further comprises the step of impregnating said mold element with colloidal silicic acid.
 5. A process for producing a metallurgically slow reacting mold comprising the steps of mixing together pulverized magnesium carbonate as a mold material and colloidal hydrolyzed silicic acid as a bonding medium to form a dipping mass wherein the quantity ratio of magnesium carbonate to silicic acid amounts to about 10 to one; producing a coating of said mass on a destructable pattern to coat said pattern with at least one layer of said mass; forming a green casting mold with said layer forming an inner surface thereof; and subsequently burning the green casting mold a length of time from 8 to 12 hours to completely bond the active magnesium oxide formed during burning with said silicic acid to form a magnesium oxide and forsterite mold from said layer.
 6. A process for producing a metallurgically slow reacting mold comprising the steps of mixing together pulverized Magnesium carbonate as a mold material and colloidal hydrolyzed silicic acid as a bonding medium to form a mixture wherein the quantity ratio of magnesium carbonate to silicic acid is sufficient to have the active magnesium oxide produced upon subsequent subjection of said mixture to burning completely bond said silicic acid as forsterite, producing a coating of said mixture on a destructable pattern with at least one layer of said mixture thereon, forming a green casting mold of said coating and pattern, and thereafter burning the green casting mold to remove said pattern and form a magnesium oxide and forsterite mold from 8 to 12 hours from said layer, said burning being of a length of time sufficient to completely bond said silicic acid in said active magnesium oxide produced during said burning step.
 7. A process for producing a metallurgically slow reacting mold consisting of mixing together pulverized magnesium carbonate as a mold material and colloidal hydrolyzed silicic acid as a binding medium to form a dipping mass; and dipping a destructable pattern into said dipping mass to coat said pattern with a layer of said mixture and form a green casting mold; subsequently burning the green casting mold at a temperature of from 800* to 1,000* C for a length of time of from 8 to 12 hours to remove said pattern and to form a magnesium oxide and forsterite mold from said layer, wherein the temperature and burning time are inversely proportional to each other and wherein the quantity ratio of magnesium carbonate to silicic acid in said dipping mass and the burning are sufficient to completely bond the active magnesium oxide formed during burning with said silicic acid as said forsterite. 